1. Field of the Invention
This invention relates generally to valves used on large containers of liquids, such as on the holds of river barges, for example, to enable contents of the container to be spilled from the top to avoid development of excessive pressure in the container at any time in general, and during filling operations in particular.
2. Description of the Prior Art
A commonly used approach to limiting internal pressure is to employ a valve which comprises a lid covering a circular opening in the top of a cargo tank. The lid has a hole in the center received on a guide post fixed in the tank, and which guides the lid as it opens and shuts. It is the weight of the lid which keeps it sealed on the opening in the top of the cargo hold. Several problems are encountered with such an arrangement. For one thing, the friction between the lid and the opening and the lid and the shaft causes the valve to be inaccurate with regard to the internal pressure at which the valve is intended open. Also, it is difficult to assure adequate spill volume without attendant and excessive internal pressure. Also, variations in response to internal pressure may occur as a result of accumulation of dirt, ice or other foreign matter on the valve guide. Also, accumulations of snow or ice on the lid may increase its weight to the point where excessive internal pressure will be reached before the valve will open.
It is an object of the present invention to overcome one or more of the shortcomings of the prior art as described above.